Tire retreading machines, commonly referred to as buffers, are used in the process of retreading tires to remove the existing tread surface from a tire, to prepare the remaining surface by cutting, and to buff the receiving surface to promote better adhesion prior to applying a new tread.
Typical tire retreading buffers include a rasp, which typically may be comprised of two mounting plates referred to as a pinside and a topside plate respectively. It is further comprised of locating or mounting pins extending between the hub mounting plates. One end of each mounting pin may be fixed to the pinside mounting plate, and the other end of each pin is received in a hole in the topside plate, which may be removed for blade replacement. The end portions of the pins received in the mounting plates are straight, but the center portions, on which the rasp blades are received, are curved for convex hubs. Cutting blades are received on the curved central portions of the mounting pins, and separating spacers are located between the blades to form stacks or “sections”. The rasp assembly is driven in rotation (typically about a vertical axis) by an electrical motor. Usually, the axis of rotation of the hub is vertical, while the axis of the tire is horizontal. When the outermost portion of a rasp assembly (i.e. the cutting edges on the peripheries of the blades) come into contact with a revolving tire tread surface, the old tread is removed from the tire by means of the rasp's cutting and abrading action.
There are different types of convex hubs, which differ principally in the orientation and number of blade sections comprising the hub.
Hubs are typically comprised of five sections (wherein each section occupies a 72° circumferential segment) or six sections (wherein each section occupies a 60° segment). Thus, each section of the assembled rasps is comprised of alternate blades and spacers arranged side-by-side. Furthermore, each section of blades and spacers are angled or offset relative to a plane perpendicular to the axis of rotation of the hub to form a cut angle, so that each blade in rotation has an axial cutting swath greater than the width of the blade. The width of the cut swath depends on the cut angle.
There are two primary arrangements of adjacent sections of blades when proceeding circumferentially about the hub. One arrangement has the blades and spacers of every section inclined at an angle relative to a plane perpendicular to the axis of rotation (the “cut angle”). The blades of all stacks are generally parallel. When these sections are diagrammatically unwound or “peeled” away from the circular outer shape to form a flat plane for illustration purposes, and looking in a radial direction (i.e. inwardly toward the axis), adjacent sections form a sawtooth profile or configuration.
A second arrangement has the blades and spacers similarly angled to the radial plane with the blades being parallel; however, the blades of adjacent sections (moving circumferentially about the hub) are angled in opposite axial directions. Thus, the magnitude of the cut angles is the same, but the directions are opposed. When this configuration is unwound from cylindrical for illustration so that all cutting edges be in a single plane, adjacent sections form alternate peaks and valleys. This type of arrangement may be referred to as a “chevron” arrangement because when proceeding about the periphery of the hub, alternate peaks and valleys (i.e. left-facing and right-facing “V's”) are formed. It will be observed that even though the blades of adjacent stacks face different directions, the magnitude of the cut angle of a blade remains the same for all blades. For example, the blades of all odd number stacks face in one axial direction, and blades of even numbered stacks face right in the opposite axial direction (with reference to the direction of cutting motion) relative to the radial plane (or perpendicular plane), but the cut angle is the same magnitude for all blades, though in alternate axial directions.
It is generally known that the sawtooth arrangement of blade sections provides a more aggressive cutting action. That is, more material is cut away from the tread surface in a shorter time, all other factors being equal. However, one advantage to the “chevron” arrangement is that a better texture of the tread-mounting surface is provided, and that is an important consideration in achieving reliable retreads which will stand up in use. Persons familiar with rasp blades for tire re-treading will appreciate that typically, for each cutting edge, there is a corresponding buffing edge or surface. In the past, a chevron stack formation was formed only from an even number of stacks.
There are also variations in the manner in which the blades are arranged in the hub as one looks from the side in a radial or tangential direction—i.e. toward a plane parallel to the axis of rotation, but at right angles to the line of sight. This is referred to as the “profile” of the hub. The present invention relates to a so-called “convex” hub, referring to the profile of the blades. In past arrangements of convex hubs, the blades are mounted on mounting pins curved in the center, and the blades are arranged parallel to one another along the axial direction such that the distance from the axis of rotation to the cutting edge varies for corresponding points on adjacent blades as one proceeds along the longitudinal edge direction of a blade. In the case of prior convex hubs, this distance increases progressively as one proceeds axially from the two side mounting plates of the hub toward the center of a stack, thus forming a convex cut profile. The cutting edges of the two center blades may be at the same axial distance.
A convex profile stack arrangement has one drawback in that in the stack of blades the most outwardly located blade—and the progressively inwardly located blades—are located at different radii from the axis of rotation, while the actual radius of the cutting edge all blades is the same. It will be understood by those skilled in the art that rasp blades of this type are comprised of a number of small teeth, typically arranged to alternate side of the base plate of the blade. By “small” it is meant that each tooth has a cutting edge which is generally straight and has a width in the range of about 0.050-0.080 inches. As the blade rotates, these cutting edges define a “cut profile”, as that term is used herein, and which may be observed if a flat sheet of material (representing a radial plane or section of a tire) is moved toward a rotating hub.
The contact surface of a tire is curved conversely in a direction parallel to its axis of rotation to form a “crown”, and in preparing the contact surface for retreading, the hub is passed with its axis perpendicular to the axis of the tire so that the convex cut profile of the hub engages the convex contact surface of the tire and moves across it with the axis of rotation of the tire and the axis of rotation of the hub substantially perpendicular. Thus, any deviation of the concentricity of the cutting profile of the blades of the hub will result is less than ideal cutting/buffing of the tire retread surface.
Thus, when the blades of a stack are all parallel and arranged in a convex cut profile at the desired cut angle, the cutting edges are not truly circular for all blades, and the deviation from true circular varies from blade-to-blade in each stack due to the convex arrangement of identical, parallel blades, and the slight inclination of blades caused by the cut angle. It is generally understood that for a uniform cutting action, the radius of curvature of the blade cutting edge is preferably centered on the axis of rotation of the hub so that the cutting profile of each blade lies along an arc centered on the axis of rotation. The differences between the radius of curvature of a blade, and a radius the cutting profile of the blade (measured at blade axis or center) is referred to herein as a deviation in concentricity.